Industrial fields of automobiles and household electric appliances request for the reduction of production cost and the increase in productivity. Particularly in a press-forming process, the productivity increase has been promoted through the shortening of cycle time by speed increase and the extension of operation time. In that high level productivity, since the temperature increase in mold induces variations of press-forming conditions, there appear problems of generation of cracks and wrinkles, thus increasing in press-rejection rate.
As for the steel sheets for automobiles, occupied by press-forming steel sheets, there has been increasing the requirement to satisfy both the strength increase of steel sheets for improving safety and the work-saving in press-forming process including the reduction in the number of parts through integration of parts. To respond to the request, the steel sheets for press-forming are also required to have sufficient allowance in press-forming as well as the high formability.
To increase the press-formability and to increase the allowance, cold-rolled steel sheets using Ti—Nb-base very low C steels were developed, as disclosed in JP-B-7-62209, (the term “JP-B” referred to herein signifies “Examined Japanese Patent Publication”), and JP-B-47796, which sheets have already been supplied to automobile manufacturers. Along with the improvement of material qualities, however, the forming conditions of the manufacturers have become stricter than ever. As a result, under recent press-conditions, steel sheets of the above-described Ti—Nb-base very low C steels give a problem of generation of press-rejection rate. With high strength steel sheets, also the frequency of press-rejection increases along with the widening of application components of that kind of steels.
In addition, the high strength galvanized steel sheets which undergo press-forming are requested to have deep-drawing performance and to have non-aging property to suppress generation of stretcher-strains. In the past, to improve the deep-drawing performance and the non-aging property, there were developed high strength steel sheets based on IF steels in which the contents of C and Mn are minimized, and Ti, Nb, and the like are added to fix harmful C and N as carbo-nitrides. The IF steels, however, have a problem of high sensitivity to the secondary working brittleness. Furthermore, since the grain boundary strength relatively decreases with the increase in the strength of the steel sheets, the secondary working brittleness likely occurs. Accordingly, the development of high strength steel sheets having excellent deep-drawing performance should emphasize the improvement of resistance to secondary working brittleness as a critical issue. There are several technologies to increase the resistance to secondary working brittleness while maintaining the characteristics almost equal with those of IF steels, as disclosed in JP-B-61-32375, JP-A-5-112845, (the term “JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”), JP-A-5-70836, and JP-A-2-175837.
However, the steels of JP-B-61-32375 and JP-A-5-112845 increase the resistance to secondary working brittleness by leaving solid solution C therein, so that there is a problem of aging when the steels are allowed to stand in a relatively high ambient temperature, such as in summer, for a long period. The steels of JP-A-5-70836 increase the resistance to secondary working brittleness by the addition of B. Boron, however, segregates in grain boundaries to suppress the crystal rotation during cold-working, which hinders the development texture favorable in attaining high r value, and degrades the deep-drawing performance. The steels of JP-A-2-175837 increase the resistance to secondary working brittleness owing to the addition of Nb to bring the grain boundary shape in a saw-teeth shape, thus making grain boundary fracture difficult. Those types of characteristics, however, make the working difficult.
As for the press-formability of cold-rolled steel sheets, investigations have been conducted mainly from the standpoint of deep-drawing performance and of stretchability. Regarding the deep-drawing performance, increase in r value is focused on, as described in JP-A-5-58784 and JP-A-8-92656. When, however, the cold-rolled steel sheets described in JP-A-5-78784 and JP-A-8-92656 are applied to side panels which are formed mainly for stretching, the punch-shoulder portion where a flat deformation stretch forming is conducted may induce fracture owing to insufficient propagation of strain. To that type of fracture occurred during that kind of stretch-forming, no appropriate action can be given because the increased strength of the materials does not allow to give evaluation by the total elongation and the n value, which are applicable in conventional mild materials.